The present invention relates in general to the processing of materials, and more particularly to substrates for electronics, optics, or optoelectronics. Described are a method and device for automatically determining the surface quality of a bonding interface between two wafers. A bonding wave is automatically detected at a predetermined measuring site to determine when bonding occurs at the measuring site, and the bonding speed is automatically calculated based on a location of the measuring site and at least one other predetermined site.
Devices for characterizing the bonding speed between two wafers are known. In particular, when at least one of the wafers is formed from a semi-conductor material such as silicon, then the bonding speed between two wafers of material can be characterized in a known manner. Such wafers generally have a disk-shaped outline and are typically of the order of few hundred microns thick. The wafers can be bonded together to form multi-layer structures, and the two bonded wafers may be of different materials.
Specifications regarding the quality of such wafers can be extremely exacting, in particular as regards wafers used to form substrates of the above-mentioned type. When bonding wafers together, the surface quality specifications are even more stringent, in particular with regard to the surface roughness of the wafer and to the absence of impurities on that surface. It is thus particularly important that high performance means be available to characterize the surface quality.
One known means for characterizing the surface quality of two wafers (at least one wafer may be formed from a semiconductor material such as silicon, and the two wafers having the same disk-shaped outline) operates as follows. The respective surfaces of the wafers are brought into contact to form a stack having the general shape of a disk, and the two wafers are thus placed side by side. The stack formed by the two wafers is then compressed at any point on the stack. Next, the time required to establish a bond between the two wafers following the compression is measured. This time measurement is termed the “bonding time”.
“Bonding” actually corresponds to forming hydrogen bonds between the surfaces of the two wafers (more accurately, between OH bonds on the respective surfaces of the two wafers). To determine the bonding speed, an operator records the time between the initial compression and the arrival of a bonding wave at the other extremity of the stack. Infrared radiation is transmitted from a source placed beneath the stack, and the operator observes the stack from above to observe the bonding wave through the wafers (using means such as a screen that reconstitutes the image provided by an infrared camera located above the stack). The bonding speed is representative of the surface quality of the wafers. A high bonding speed indicates good surface quality, while a slower bonding speed indicates the presence of defects and/or impurities on the wafer surface.
The surfaces of wafers leaving a fabrication facility may exhibit several defects and residues which are both chemical in nature (principally OH bonds with superficial wafer particles) and mechanical in nature (typically, roughness defects, the presence of abrasive particles and superficial metallic contaminants). To improve the surface quality, measurement of the bonding speed is generally preceded by cleaning the wafers. Cleaning the wafers may include immersing them in succession in at least one basic dip. This step is intended to develop the hydrophilic characteristics of the wafer, by creating OH type bonds at the surface of the wafer. Next, an acid dip is used to remove contaminating elements (in particular metals) from the wafer surface, which contaminants may have been introduced by preceding wafer processing steps (in particular the basic dip). Thus, a method for characterizing surface quality of wafers is known.
However, the disadvantage of this method is that an observer must manually perform the different steps (placing the wafers on the measuring device, performing the initial compression, followed by observing the bonding wave, timing to determine the bonding speed, etc.). Further, this conventional method requires carrying out measurements of the bonding speed on a dedicated manual device. Therefore, wafer bonding cannot be carried out automatically.
Devices exist which automatically bond wafers, where the goal is to perform operational bonding rather than to characterize the surface quality. An operator could employ such automatic bonding devices to carry out a measurement of the bonding speed in conjunction with bonding, thus producing information regarding the surface quality of the wafer. It is currently not possible to contemplate such a use of automatic bonding devices by an operator because the micro-mechanisms present above and below a support for positioning, aligning and bonding the wafers do not allow an operator to view the bonding wave. Currently, therefore, a bonding wave can only be characterized in a manual manner using a dedicated device. Consequently, such operation substantially limits the rates that can be obtained.